The purpose of these investigations is to define the relationship of human papovavirus JC to BK and SV40 papovaviruses, and to determine the role of JC virus nucleic acid in the development of malignant brain tumors. Comparison will be made between JC, BK and SV40 viral DNA by hybridization analysis, and relatedness between viral genomes will be correlated with tumors induced by either JC or SV40 viruses in hamsters. Both intact JC virus DNA and fragments generated by restriction endonuclease digestion will be used as probes to detect and quantitate representation of the JC genome in DNA from established hamster tumors in vitro and in vivo, by molecular hybridization techniques. Finally, JC viral DNA will be used as a probe to determine the possible role of the ubiquitous human tumors of brain or other organs.